There have been made a large number of proposals on processes and catalysts for producing methacrylic acid by vapor phase catalytic oxidation of methacrolein. In one of these proposals, there has been disclosed a process using an organic substance such as a cellulose, a polyvinyl alcohol or the like in preparation of a catalyst, for the purpose of controlling pores (the specification of British Patent Laid-Open Application No. 2037604). As an invention for controlling pores, there is the invention disclosed in Japanese Patent Unexamined Publication No. 60-239439. These catalysts, however, have defects such as unsatisfactory reaction results, a large decrease of catalytic activity with the lapse of time, and a troublesome after-treatment, and there is a desire to seek further improvement in them for their employment as industrial catalysts.